(1) Field of the Invention
The present invention relates to an improved heat-sensitive recording material employing a colorless or pale-color leuco dye and an acidic material such as a phenolic compound or an organic acid capable of making said leuco dye develop color when heated, which is intended to obtain a colored image of high density developed by recording with a low heat energy at a low temperature and to effect smooth recording free from generation of residue of any adhesive substance and sticking thereof onto the thermal head by virtue of making the heat-sensitive color forming layer contain at least one of the compounds expressed by the following general formulas (I), (II) and (III): ##STR4## (wherein X.sup.1 represents a hydrogen atom or a halogen atom; Y.sup.1 represents an alkyl group, an alkoxy group, a halogen atom, a benzyl group, a hydroxyl group, a carboxyl group or an allyl group; m and n are each an integer ranging from 1 to 3, respectively; and X.sup.1 and Y.sup.1 may be either identical or different from each other when m or n is an integer of 2 or more;) ##STR5## (wherein X.sup.2 represents a hydrogen atom or a halogen atom; Y.sup.2 represents a hydrogen atom, an alkyl group, an alkoxyl group, a halogen atom, a benzyl group, a hydroxyl group, a carboxyl group or an allyl group; m and n are each an integer ranging from 1 to 3, respectively; and X.sup.2 and Y.sup.2 may be either identical or different from each other when m or n is an intger of 2 or more;) ##STR6## [wherein X.sup.2, Y.sup.2, n and m are respectively the same as in the foregoing general formula (II).]
2. Description of the Prior Art
Heat-sensitive recording material consists essentially of a support such as paper and a heat-sensitive color forming layer capable of developing color when heated, which layer is formed on the surface of said support. For the purpose of heating on this occasion a thermal printer having a thermal head therein or the like is usually employed, and image recording is effected according to heat signals.
The heat-sensitive recording method employing such a recording material as described above is prevalent not only in copying books, documents and so forth, but in recording for various output units, e.g., electronic computers, facsimiles, meters, etc., and as to the recording material for use in this method, it is well known in this field of industry that recording materials prepared by forming a heat-sensitive color forming layer containing a colorless or pale-color leuco dye having a lactone, lactam or spiropyran ring and an acidic material, e.g., organic acid or a phenolic compound on a support display a distinct color tone and superior contrast. However, these heat-sensitive recording materials are recently desired to be so improved as to be fit for high-speed operation of the output units and prolonged printing.
Heretofore, in order to improve the heat-sensitivity of recording materials there have been adopted methods wherein acetoacetic anilide is employed (cf. Japanese Laid-open Patent Application No. 106746/1977), a method wherein particles of at least one of the dye and color developing agent are to be impregnated with a heat-meltable substance having a melting point ranging from 60.degree. to 200.degree. C. (cf. Japanes Laid-open Patent Application No. 48751/1978), a method wherein alkyl biphenyl and substituted biphenyl alkane having a melting point ranging from 60.degree. to 200.degree. C. are employed (cf. Japanese Laid-open Patent Application No. 39139/1978), method wherein saturated fatty acid having 12 to 22 carbon atoms is to be added to fatty acid amide (cf. Japanese Laid-open Patent Application No. 139740/1979), etc., but any of these methods leaves something to be desired in respect of the effect and a further improvement is desired. Moreover, the foregoing thermosensitizers are defective in that use of them in large quantities would cause adhesion of residue or `sticking` onto the thermal head, resulting in remarkable deterioration of the traveling property. Therefore, in the case where it is required to obtain a distinct image of high density invariably even when used for a long term by applying especially a low heat energy at a low temperature, such as in high-speed facsimile transmission, it is necessary to reduce the color-developing temperature and also select a sensitizer having thermo-sensitivity which does not generate residue of adhesive substance or the like that is apt to stick onto the thermal head.